The present invention relates generally to bone screws, and more particularly, to a new and improved bone screw having an intermediate helical spring portion, wherein a resorbable material is disposed within the spaces of the helical spring portion when the bone screw is in a stressed state, so as to provide various rates of compliant fixation.
The use of orthopedic fastening devices, such as bone screws, has greatly aided the medical field in the treatment of bone fractures, as well as enabling the ever increasing use of orthopedic implants and orthopedic appliances. With respect to the treatment of bone fractures, it is sometimes generally necessary to surgically reposition the fragmented bone members in an anatomically acceptable orientation, and then fasten the bone members together in order to facilitate the healing process. Bone screws are typically employed in stabilization procedures used to treat bone fractures.
When a bone screw is employed, either to fasten two or more bone members together or to secure an orthopedic appliance (e.g., bone plate) to a bone surface, and the bone screw is tightened, initially, tension in the screw is relatively very high, and holds the bone members together. However, bone is a viscoelastic material and undergoes a phenomenon known as stress relaxation immediately after torque has been applied to the bone screw. The stress relaxation response is quite pronounced and causes immediate and rapid reduction in the bone screw tension and, hence, the force holding the bone: members together. Furthermore, after a conventional bone screw is tightened, and the bone member is laterally displaced, as by bending, the rigidity of the bone screw causes the surrounding bone to fail because the bone has lower strength and stiffness than the bone screw. This can lead to failure of the fixation and eventual non-union or misalignment of the bone members at the fracture site.
One approach to overcoming this problem has been the use of dynamic tension bone screws. U.S. Pat. No. 4,959,064 to Engelhardt describes an example of a dynamic tension bone screw. A conventional bone screw, having a proximal head portion and a distal threaded shank, is modified by milling a length of the shank portion to remove the outermost peak of the threads formed on the shank. This results in an intermediate segment having a smaller diameter than the threaded shank. Next, an elongated bore is formed, symmetrical with the longitudinal axis of the bone screw and having a diameter slightly larger than the root diameter of the threads on the shank. This perforation of the surface of the bone screw in a spiral fashion along the thread root results in the formation of a tension spring. It should be noted that both the threaded shank and the tension spring are oriented in the same general direction (e.g., both are either xe2x80x9cright-handxe2x80x9d oriented). The bore extends as far as the distal end member into which is suitably formed, symmetrical about a longitudinal axis of the bone screw, a tool receiving recess.
When it is desired to rejoin a pair of bone fragments, a pair of aligned bores are formed respectively in the two bone fragments. Thereupon, the modified bone screw is inserted into the first (i.e., proximal) bore, and a suitable tool, such as a hexagonal drive wrench, is inserted into the recess to rotate the distal end member around its longitudinal axis, and therefore, also the rest of the modified bone screw. With continued insertion of the modified bone screw, the distal end member eventually enters and advances along the second (i.e., distal) bore of the distal bone fragment. As the distal end member continues to advance along the distal bore, the head member moves into engagement with the outer surface of the proximal bone fragment such that continued advancement of the distal end member causes the spring to stretch beyond its relaxed condition (due to the spring having the same orientation as the threaded distal end member). As the bone fragments undergo stress relaxation, the modified screw similarly relaxes, while continuing to hold the fragments together due to the remaining tension in the spring portion.
While the modified bone screw described by Engelhardt was an improvement over conventional bone screws, with respect to conserving tension in the screw over time as stress relaxation occurred, there still exists a need for a new and improved bone screw, and method of making same, that combines the best feature of conventional bone screws, i.e., high initial tension in the bone screw, with the best feature of dynamic tension bone screws, i.e., conservation of moderate tension in the bone screw over time, while avoiding the worst feature of conventional bone screws, i.e., rapid decrease in tension in the bone screw over time, and the worst feature of dynamic tension bone screws, i.e., relatively lower initial tension in the bone screw as compared to conventional bone screws.
In accordance with a first embodiment of the present invention, a bone screw is provided, comprising (1) a distal portion having a threaded surface thereon, the threaded surface having a first orientation, (2) a proximal portion, and (3) an intermediate portion comprising a compressive member integral with said distal and proximal portions, the compressive member having a second orientation opposite that of the first orientation.
In accordance with a second embodiment of the present invention, a bone screw is provided, comprising: (1) a distal portion having a threaded surface thereon, (2) a proximal portion, (3) an intermediate portion comprising a compressive member integral with said distal and proximal portions, the compressive member having an area defining at least one space between adjacent surfaces of the compressive member, and (4) a resorbable material disposed within the at least one space of the compressive member.
In accordance with a third embodiment of the present invention, a bone screw is provided, comprising: (1) a distal portion having a threaded surface thereon, the threaded surface having a first orientation, (2) a proximal portion having a recess formed therein for receiving a member capable of permitting the insertion of the bone screw into a bone member, (3) an intermediate portion comprising a compressive member integral with said distal and proximal portions, the compressive member having a second orientation, the compressive member having an area defining at least one space between adjacent surfaces of the compressive member, and (4) a resorbable material disposed within the least one space of the compressive member when the compressive member is in a stressed state.
In accordance with a fourth embodiment of the present invention, a method of making a bone screw is provided, comprising: (1) providing a member, (2) forming a threaded surface on a distal portion of the member, the threaded surface having a first orientation, and (3) forming a compressive member adjacent to the threaded surface, the compressive member having a second orientation opposite that of the first orientation, wherein the compressive member has an area defining at least one space between adjacent surfaces of the compressive member.
In accordance with a fifth embodiment of the present invention, a method of making a bone screw is provided, comprising: (1) providing a member, (2) forming a threaded surface on a distal portion of the member, the threaded surface having a first orientation, (3) forming a compressive member adjacent to the threaded surface, the compressive member having a second orientation opposite that of the first orientation, the compressive member having an area defining at least one space between adjacent surfaces of the compressive member, and (4) applying a resorbable material onto the least one space of the compressive member.
In accordance with a sixth embodiment of the present invention, a method of securing a bone member to an adjacent member is provided, comprising: (1) forming at least one bore in the bone member, (2) providing a bone screw, (3) inserting the bone screw into the at least one bore. The bone screw includes a threaded surface on a distal portion thereof, the threaded surface having a first orientation, a compressive member adjacent to the threaded surface, the compressive member having a second orientation opposite that of the first orientation, the compressive member having an area defining at least one space between adjacent surfaces of the compressive member, and a resorbable material disposed within the least one space of the compressive member.
In accordance with a seventh embodiment of the present invention, a method of securing a bone member to an adjacent member is provided, comprising: (1) forming at least one bore in the bone member, (2) providing a bone screw, and (3) inserting the bone screw into the at least one bore. The bone screw includes a threaded surface on a distal portion thereof, the threaded surface having a first orientation, and a compressive member adjacent to the threaded surface, the compressive member having a second orientation opposite that of the first orientation, wherein the compressive member has an area defining at least one space between adjacent surfaces of the compressive member.